FIELD OF THE INVENTION
The invention relates to a circuit configuration for generating a boosted output voltage, having a p-channel MOS transistor and a boosting capacitor.
Such circuit configurations, which generate an output voltage that is boosted above the applied supply voltage, are employed in a large number of semiconductor circuits, in particular in semiconductor memories. A single-transistor memory cell of a semiconductor memory, for example of a DRAM, includes a storage capacitor for storing an information bit as well as a transfer transistor through which access is made to the memory cell by the storage capacitor being connected to a word line through the main current path of the transfer transistor. In order to store an information bit at the level of the complete level of the supply voltage in the cell, it is necessary for the gate potential at the transfer transistor to lie above the supply voltage by that transistor's own threshold voltage. However, due to the usually small channel width of the transfer transistor and the high substrate-source voltage thereof, the threshold voltage is relatively high.
A circuit configuration for generating the boosted voltage for driving a transfer transistor in a semiconductor memory is described in Published European Patent Application 0 635 837 A2. A charge pump shown therein contains a p-channel MOS transistor through which a charging capacitor on the output side is charged by a boosting capacitor. The charging operation is controlled by an oscillator and is carried out continuously, with the output voltage being permanently present. Therefore, additional switches are necessary in order to forward the boosted voltage to the transfer transistor. While the gate terminal of the p-channel MOS transistor is at 0V, one of the terminals of its main current path is already connected to the output voltage which is applied to the charging capacitor and is boosted beyond the supply voltage. The gate oxide of the p-channel charging transistor is exposed to elevated voltage stress. Furthermore, the voltage present between the terminals of the main current path of the load transistor changes its direction during the pumping operation. Therefore, special measures are described for avoiding current flow in the doping well in which the charging transistor is disposed.